geometric design of highway

1. GeometricDesignofHighways

2. Introduction
 It refers to the dimensioning of the elements of
highways, such as vertical and horizontal curves, cross
sections, truck climbing lanes, bicycle paths, and parking
facilities etc.
 Takes into concern the engineering principles as well as
the social and environmental impacts of the highway
geometry on the surrounding facilities
Necessities
• To decrease the cost of construction
• To Decrease the cost of operation
• To maintain consistency of traffic flow
• To ensure safety
• To maintain asthetics of highway allignment

3. 3
Objectives of Geometric Design
• To incorporate various physical features
of road alignment as per design standard
• To provide optimum efficiency in traffic
operation with maximum safety at reasonable cost.
• To incorporate human behaviors
• To promote the environmental benefit
• To provide a basis to evaluate the
construction of the proposed highway.

4. Scope of Geometric Engineering
1. Elements of Cross-Section
 Typical Cross Section
 Traffic lane, Carriageway, Shoulder, Median-strips, Right of Way,
Side Slope
 Camber
 Super elevation
2. Elements of Horizontal Alignment
 Tangent
 Horizontal Curves and its Elements
 Transition Curve and its Elements
 Extra widening of horizontal curves
 Laybys

5. 3. Sight Distance across the road
 Stopping Sight Distance
 Intermediate Sight distance
 Overtaking Sight distance
 Decision Sight Distance
4. Elements of Vertical Alignment
 Grade
 Vertical Curves (Summit Curve and Valley Curves)

6. Design Control Criteria
 Road Classification
 Design Speed
 Design Vehicle
 Driver Characteristics
 Traffic Volume and Composition
 Level of Service
 Social and Environmental Considerations
 Topography
 Economy
 Safety

7. 7
• National Highway
• Feeder Road
• District Road and Village Road
• Urban Road
Road Classification
1. Administrative/Functional Classification

8. 8
National Highways
• Main arterial roads connecting East
to West and North to South
• Longer distance travel, higher
the inter- community mobility.
• designated by letter “H” followed by
a two- digit number.
Transportation Engg. (IOE MSc.)

9. 9
Feeder Roads
• Connect District Headquarters, Major economic
centers, Tourism centers to National Highways or
other feeder roads.
• They are designated by letter “F” followed by 3-digit
number
Transportation Engg. (IOE MSc.)
District Roads
• Roads connecting district headquarters to village
or village to village
• Serves rural area of production , market centers
service centres etc.
• Speed 50-60kmph (district road) , 40-50kmph
(village roads)

10. 10
Urban Roads
• The roads serving within the urban municipalities.
Except highways and feeder roads
• Designed to maintain flow as well to provide
modern facilities and also to maintain the city
aesthetics
• Design Speed 40-50kmph
Transportation Engg. (IOE MSc.)
In Nepal(Before Federalism)
1. National Highway + Feeder Roads = SRN = Department of Roads (DOR)
• Has published Nepal Road Standard (NRS)
2. District Roads + Urban Roads = LRN
• District Roads = Department of Local Infrastructure and Agricultural Roads
(DoLIDAR) and गा.वि.स.
• Has Published Nepal Rural Road Standard (NRRS)
• Urban Roads = Municipalities and Department of Urban Development and
Building Construction (DUDBC)

11.  Nepal Rural Road Standard (NRRS) has further classified
district roads into two categories
 District road core network : connects villages to district
headquarter of to major economical centeres
 Rural Road : Connecting Village to Village
11
2. Technical Classification
Class ADT (PCU)
Class I 20,000 PCU
Class II 5000 – 20000 PCU
Class III 2000 – 5000 PCU
Class IV <2000 PCU

12. 12
Design Speed
• A selected speed to determine the various
geometric features of the roadway.
• The maximum safe speed that can be maintained
over a specified section of highway
• Design speed shall be taken as the 98th percentile
value for traffic on the roadway the reference design
speed depends on
-- The functional classification of the
highway,
– The topography of the area (level, rolling,
and mountainous terrain)
– the land use of the adjacent area

13. Design Speed as per NRS 2070
Transportation Engg. (IOE MSc.)
13

14. 1
4
Design Vehicle
• Largest design Vehicle likely to use facility with
considerable frequency as a design vehicle
• Design of critical features such as radii of
horizontal curve, radii at intersections with
respect to a vehicle with special characteristics

15. 15
Source: J. H. Banks Introduction to Transportation Engineering
Relationships between vehicular and facility
characteristics (1/2)
Vehicular characteristics Related facility characteristics
Length Park stall length
Transit station platform length
Width Lane width
Parking stall width
Lateral clearance
Height Vertical clearance
Minimum vertical curve length
Wheelbase (turning radius) Lateral clearance on curves
Intersection edge radii
Transportation Engg. (IOE MSc.)

16. Vehicle Dimension as Per NRS 2070
39
Transportation Engg. (IOE MSc.)

17. Topography
17
• As the topography becomes more extreme the design
standards become more flexible
• The different types of topography are decided based
on the cross slope and are as follows

18. 18
Driver’s
Characteristics

19. 19
Driver’s
Characteristics
• The driving task – control, guidance and navigation.
• The various parameters like perception reaction time ,
psychological extra widening etc. are directly dependent on
the drivers behavior
• Geometric Design must incorporate
• Older drivers – capabilities and needs of older road users
• Information handling (collects information, make
numerous decisions, and perform necessary control
actions and act timely i.e. - reaction time)
• Driver error (insufficient experience, inappropriate risk
taking, poor glare recovery, fatigue, sleep deprivation or
prolonged exposure to monotonous environments)
• The guidance task (lane placement, car following,
passing maneuvers, merging & diverging, response to
traffic control devices )
Transportation Engg. (IOE MSc.)

20. 20
Level of Service
LOS
A Free flow Completely unimpeded maneuver
B Reasonable
Free
Maneuver within traffic stream is slightly
restricted
C Stable Maneuver within traffic stream is visibly
restricted
D Reaching
Unstable
Speed decrease and drivers are in
discomfort psycologically
E Restricted Vehicles are closely spaced, slight diruption
result long queuing
F Congested Greater than capacity of road

21. 21
Level of Service
• Recommended to adopt a LOS “B” for the design capacity
of roads.
• Under this condition, traffic will experience congestion
and inconvenience during some of the peak hours, which
may be acceptable.
• At the level of service B, volume of traffic will be around
45 percent of capacity under mixed traffic condition

22. 22
Social & Environmental Consideration
• Emits pollutants and transmits noise
• Pollutants emitted impact on land uses
adjacent to highways
• Noise and pollutant affects distance to the
highway from a residence or workplace
• Impact on natural and historic assets
• road side developments

23. 23
Safety Consideration
• Highways should be designed to minimize driver decisions and to
reduce unexpected situations.
• Some measures:
– Access control
– Safe Speed limit
– Characteristics of expected drivers
– Design of Curves, sufficient Radius
– Wide median or Median barriers
– Shoulder width
– Sight distances
– Treatment of obstacle (Remove, relocate or reduce severity)
– Intersection Treatment (Channelization, refuse islands)

24. Typical Cross Section of Highway

25.  Traffic lane (TL): It is the strip of the carriageway occupied by vehicles
moving in a single stream along the road.
TL= (width of vehicle + safety clearance on either side)
 Carriage Way (CW) : It may be defined as that strip of road which is
constructed for the movement of vehicular traffic. The carriageway
generally consists of hard surface to facilitate smooth movement and is
made of either hard bituminous treated materials or cement concrete. It is
also called Pavement width. By definition
 Shoulder:
 It is the portion of roadway on either side which is periodically used by vehicles
during crossing, overtaking and parking maneuvers.
 Shoulder is an important element of rural road.
 Laybys are the intermittent shoulders provided in hill roads. Laybys are provided
as continuous shoulder cannot be provided in hill roads.
Required Width
2 to 4 Lane 4 to 6m on either side
Single / Intermediate Lane 3 to 5 meter
Minimum 0.75 on each side
In practice in Nepal; width of shoulder = 0.5 to 1.5 m

26.  Advantages of Shoulder:
i. Provides space for parking vehicles during repair etc
ii. Capacity of road increased because of frequently
available opportunity for overtaking
iii. Sufficient space available for parking vehicles on rest
iv. Provides space for fixing traffic signs away from the
pavement
v. Shady trees can be grown up away from the
pavement
vi. Provides sufficient space for confidence in driving
vii. Proper drainage strengthen the life of the
pavement
viii. Increased effective width of carriageway
ix. Lateral clearance increases the sight distance

27.  Side Slope of fill or cut:
 1:1 to 1:1.5 in cutting
 1:1.5 to 1:2 in filling
 Right of Way (ROW) or Land Width:
 The strip of land on either side of road from its centre line
acquired during road development and which is under the
control of road authority (DoR Nepal)
 i. National Highway =25 m on either side
 ii. Feeder Roads = 15 m on either side
 iii. District Roads = 10 m on either side

28.  Functions
 Right of way may be used for the following purposes:
i. to accumulate drainage facilities
ii. to provide frontage roads/driveways in roads with
controlled access
iii. to develop road side arboriculture
iv. to open side burrow pits
v. to improve visibility in curves
vi. to accommodate various road ancillaries
vii. to widen the road where required in future with no
compensation for property

29. Typical Cross Section of Urban Roads

30. 1. Traffic Lane
2. Carriageway
3. Sidewalk or Foot Path: It is that portion of urban road
which is provided for the movement of pedestrian
traffic where the intensity is high.
4. Kerb : It is that element of road which separates
vehicular traffic from pedestrians by providing physical
barrier (15-20 cm)
5. Median Strip (or Traffic Separator or Central
Reservations): It is the raised portion of the central road
strip within the roadway constructed to separate traffic
following in one direction from the traffic in opposite
direction.
6. Side Drain
7. Catch Pit and Cross Pipe

31. Camber
 it may be defined as the slope of the line joining the crown
(topmost point) of the pavement and the edges of
pavement.
 Camber is Expressed either as the ration of the rise to the
half width of pavement(1:50 1:25 etc.) or as percentage (2%
4% etc.)
 Necessity/Advantages of Camber: -
 to drain of surface water quickly
 to prevent infiltration into underlying pavement layers and
sub-grade
 To give the driver a physiological feelings of the presence of
two lanes
 To improve the road appearance

32.  Disadvantages of providing heavy camber:-
 Central portion of road is excessively eroded
 Causes uncomfortable side thrust drag
 Overtaking operations may be dangerous especially in two
lane roads
 There is possibility of overturning and skidding of vehicles
 Low cost surface and shoulder will be excessively eroded
due to increase velocity of water. This may leads to
formation of cross ruts.
 Tendency of driver to travel through centre line of road. So
centre line area undergoes more wear and rear.
 Types of Camber
 1. Straight line camber Y= nX
 2. Parabolic camber Y= (2n/W) x2
 3. Composite camber

33. Numerical Example
The centerline of a two lane highway has an
elevation of 320.5m as recoreded from the L-
Profile , if the carraigeway has a camber of 2.5%
and the shoulder has a camber of 5% , calculate
the RL of
(i) The center of traffic lane
(ii) The edge of carriageway
(iii) The edge of shoulder
If the camber provided is
(A) Parabolic Camber for Road Surface
(B) Straight line camber For Both Road and
Shoulder
Take lane width as 3.5m per lane and shoulder
width as 1.5m

34. SUPERELEVATION
 The outer edge of the pavement is raised with respect to the
inner edge in order to provide a transverse slope throughout
the length of the curve. This transverse slope is known as
superelevation.
 Superelevation can be described in the form of ratio of the
rise to the width of pavement (1:15 , 1:20 etc.) or as
percentage (5% 4% etc.)
 Design Superelevation
 The value of superelevation that is required to be adopted in
the field to sustain the design speed or tha allowable speed is
called design superelevation
 The maximum/Limiting value of design superelevation are as
follows
 In Plain and rolling terrain 7%
 In Snow bound areas 7%
 In Hilly areas not bounded by snow 10%

35.  Expression for design superelevation
𝒆 + 𝒇 =
𝒗𝟐
𝟏𝟐𝟕𝑹
Calculation of Design Superelevation
 1. In order to overcome the effect of combined equation , 75% of design
speed is taken neglecting f
 𝒆 =
𝟎.𝟕𝟓𝒗𝟐
𝟏𝟐𝟕𝑹
If the calculated e is less than 0.07 (7%), the value obtained is
provided. If the valued of e exceeds 0.07, then provide maximum
superelevation i.e. e=0.07 or 7%
 3. Check the coefficient of friction developed for the selected value of e at
the full value of design speed 𝒇 =
𝒗𝟐
𝟏𝟐𝟕𝑹
− 𝒆 If the value is less than
0.15, calculated value is provided. And the given conditions are sufficient to
handle the design velocity. If the value of f exceeds 0.15, the given section
cannot sustain the design speed and given designed speed is reduced to
allowable speed.
 4. The allowable speed (Va kmph) at the curve is calculated by considering
the design coefficient of lateral friction and the maximum superelevation:
𝒗𝒂 = 𝟏𝟐𝟕𝑹(𝒆 + 𝒇)

36. Numerical Example
The design velocity of a highway is 80
Kmph , there is a horizontal curve of
radius 100m.
Calcualte the design superelevation
and check weather the velocity can
be sustained or not , if not determine
the allowable velocity
Take maximum cant as 1 in 15 and
maximum allowable coefficient of
friction as 0.15

37. Method of Providing Superelevtion
1. By Elimination of Crown

38. 2. By rotation of pavement

39. Elements Of Horizontal Alignment
 It includes tangents and curves
 Deviations in horizontal alignment are encountered due to various reasons
as:
• 1. Topography of the terrain
• 2. Restrictions imposed by property
• 3. Minimizing quantity of earthwork
• 4. Need to provide access o the particular locality
• 5. Other factors controlling highway alignment
• 6. Maintaining consistency with existing topographical features of the terrain
blending with existing topographical or other features)
• 7. Reduce mental strain produced by travelling monotonously along the straight
route.
 Curves are provided in each and every points of intersection of two straight
alignments of roads in order to change the direction.
 Provision of horizontal curves at deviation points allows the vehicle to turn
 Radius additional to the minimum radius enhances comfort to the passenger
by avoiding sudden change in direction.

41. Elements of simple circular Curve
R = Radius of Curve
BC = Beginning of curve
EC = End of curve
PI = Point of Intersection
T = Tengent length
L = Legth of curve
Δ=Deflection angle
E = External distance
M= Mid ordinate

42. Elements of Circular Curves
Radiusof Curve R=1718.9 / D
Lengthof Curve L=R/180o
Tangent: T=Rtan(Δ/2)
Chord: C=2Rsin(Δ/2)
Mid Ordinate: M =R– Rcos(Δ/2)
External Distance: E=Rsec(Δ/2) - R

43.  Numerical Example
 The deflection angle of a 4O curve is
55O25’ and the PC is located at a
station of 238+045. Determine the
length of the curve and the station of
the PT

44. EXTRAWIDENING AT THE CURVED PATH
 It is the additional width required of the
carriageway that is required on a curved path than
the width required on the straight path.
 Reasons:
 i. Rigidity of the wheel base i.e when rear wheels go
out while the front wheels are within the pavement
 ii. Preferential use of the outer lane since visibility
is enhanced when the vehicle moves along the outer
lane
 iii. More clearance between opposing vehicles
 Consists of two parts
 i. Mechanical Extrawidening(Wme) : Due to rigidity of
wheel base
 ii. Psychological Extrawidening (Wpe) : Because of
tendency of vehicle to move towards outer direction

45. Expression For Extrawidening
𝑊
𝑒 = 𝑊
𝑚𝑒 + 𝑊
𝑝𝑒
𝑊
𝑒 =
𝑛𝑙2
2𝑅
+
𝑉
9.5 𝑅
Where
We = Extrawidening
Wme = Mechanical extrawidening
Wpe = Psychological extrawidening

46. Transition Curve
 Definition
 a curve of varying radius ( R c between straight and
circular path provide in order that the application of
centrifugal force would be gradual.
 Objectives:
 i. to introduce centrifugal force gradually in order to avoid a
sudden jerk or discomfort to the passengers.
 ii. to introduce superelevation at a desirable rate
 iii. to enable the driver to turn his vehicle slowly and
comfortably.
 iv. to introduce extrawidening at the desirable rate.
 v. to fit the road alignment in a given topography and also to
improve the appearance of road.

47. VisualeffectofTransitionCurve
Smoothtransitionfrom
tangenttocurve
Thesharpcornersatthe
beginningofcurve
Source:A Policy on GeometricDesign ofHighways and Streets

48.  Types of Transition Curve:
 i. Spiral or clothoid
 ii. Bernolli's leminscate or lemniscate
 iii. Cubic parabola
 r = aθ
 a = R/2π
 Co ordinates at any point (Origin at
the intersection of transititon and
circulat curve)
 r = a sin2θ

49. Spiral is considered as ideal transition curve because
1. It satisfies that rate of change of centrifugal acceleration is
constant i.e., Ls.R = constant. Where Ls = length of transition
curve R = radius of curve.
2. The calculation and field implementation of spiral curve is
simple and easy.
3. It enhances aesthetics also.
𝐲 = 𝒙𝟑

50. Length of Transition Curve
1. Based on Rate of Change of centrifugal acceleration
 𝑳𝒔 =
𝒗𝟑
𝑪𝑹
 Where C = Rate of change of centrifugal acceleration
=
80
75+𝑣
V = velocity in kmph
2. Based on Rate of introduction of superelevation
 𝑳𝒔 = 𝒏𝒆(𝒘 + 𝒘𝒆) for rotation about inside
or outside
 𝑳𝒔 =
𝒏𝒆(𝒘+𝒘𝒆)
𝟐
for rotation about center

51.  By Empirical formula
 Plain and rolling terrain 𝑳𝒔 =
𝟐.𝟕𝒗𝟐
𝒈
 For mountainous and steep terrain 𝑳𝒔 =
𝒗𝟐
𝒈

52. Elements of transition curve
 Shift (S) =
𝑳𝒔
𝟐
𝟐𝟒𝑹
 Tangent Length
T = 𝑹 + 𝑺 𝒕𝒂𝒏
𝜟
𝟐
+
𝑳𝒔
𝟐
 Spiral angle
Φ=
𝑳𝒔
𝟐𝑹
 Length of circular curve
𝑳𝒄=
π𝑹(Δ−𝟐Φ)
𝟏𝟖𝟎
 Length of total curve
𝑳𝑻=𝑳𝒄 + 𝟐𝑳𝑺

53. NUMERICAL EXAMPLE
A two lane national highway in plain area has a curve of 525m
radius is set out to connect twi straights. The maximum
speed of the moving vehicles on this curve is restricted to 90
kmph . Transition curve are to be introduced at each end of
the curve . Calculate
a. A suitable length of transition curve is
b. The necessary shift of the circular curve
c. The Chainage at the beginning and end of the curve
Given that
Angle of intersection = 130o24’, Rate of change of centrifugal
acceleration = 0.52 m/sec3 , Chainage at point of
intersection = 1092.5 m , Lane width = 3.5m , Rate of
introduction of superelevation = 1:120 , length of wheel base
= 6.1 m

54. Sight Distance
 Visibility : The distance upto which there is clear vision in
the highway alignment is called visibility
 Sight distance : The minimum visibility requirements for
various maneuvering operations in highway is called sight
distance. The different sight distances for roadway
alignment are as follows
 Stopping sight distance (SSD) : Distance upto which the driver
shall be able to see clearly so as to avoid collision upon
seeing a stationary obstruction
 Overtaking Sight Distance (OSD) : The distance upto which
the driver shall see clearly so as to avoid collision or
uncomfortable return back if the overtaking operation has
commenced.
 Intermediate Sight Distance (ISD) : Distacne more than SSD
and less than OSD provided for better safety or when there is
chance of head on collision.

55.  Decision sight distance (DSD) : Distance upto which the driver
shall be able to see clearly so as to safely carry out complex
maneuvering operations like Transit to a ramp etc.
 Sight distance at intersection :
 The SSD for minor and major roads varies as the velocity vary
 The required sight distances are computed and the sight triangle is
ensured
 In case sight triangle cannot be ensured restrictions on minor like
slow and pass , stop and pass etc can be applied

56. Stopping Sight Distance
SSD Contains two parts
1. Perception Distance/Lagging Distance (l1) :
It is the distance travelled from the instant the
obstruction/hazard is identified to the point at which brakes
are applied,
According to PIEV theory the total reaction can be divided into
four parts Percepetion Intellect Emotion Volition . The
combined time from perception to volition is called perception
reaction time.
It is equal to desigh velocity multiplied by the perception
reaction time
2. breaking distance (l2) :
It is the distance travelled from the instant the brakes are
applied till the vehicle comes to a hault position

57. Analysis of SSD
SSD = l1 + l2
𝐒𝐒𝐃 = 𝟎. 𝟐𝟕𝟖𝒗𝒕 +
𝒗𝟐
𝟐𝟓𝟒𝜱
𝐒𝐒𝐃 = 𝟎. 𝟐𝟕𝟖𝒗𝒕 +
𝒗𝟐
𝟐𝟓𝟒ή(𝜱±𝒊)
v = design velocity
Φ = coefficient of longituditional
friction
t = perception reaction time

58. NUMERICAL EXAMPLE
Compute the minimum SSD required to
avoid a head on collision of two busses
approaching from the opposite
directions. The speed of both the buses
is 70 kmph . Assume a total perception
and brake reaction of 2.5 Sec.
Coefficient of friction is 0.4 , brake
efficiency of 50% and a grade of 4%

59. Set Back
 It is the clearance from the centerline of the highway
curve required to provide adequate sight distance
 Case I : Length of Curve(S) > Sight distance (S)
𝑚 = 𝑅 − 𝑅 − 𝑑 𝑐𝑜𝑠
α
2
R = Radius of circular curve
d = distance of center of innermost lane from center
of highway
α = angle subtended at the center by the sight
distance
α =
180𝑆
π(𝑅 −𝑑)

60. Numerical Example
A four lane divided highway
has a curve of 1000m long, a
radius of 550m, and a safe
SSD of 250m. Caclculate the
minimum set back distance
from the inner edge of the
curve to a building to ensure
safe visibility. Take pavement
width per lane as 3.5m

61.  Case II : Sight distance (S) > Length of Curve(S)
𝒎 = 𝒎𝟏 + 𝒎𝟐
𝒎𝟏 = 𝑹 − 𝑹 − 𝒅 𝒄𝒐𝒔
𝜶
𝟐
𝒎𝟐 =
(𝑺 − 𝑳)
𝟐
𝒔𝒊𝒏
𝜶
𝟐
R = Radius of circular curve
d = distance of center of innermost lane from center of
highway
α = angle subtended at the center by the curve
α =
180𝐿
π(𝑅 −𝑑)

62. Numerical Example
On a two lane highway
there is a horizontal curve
of radius 550 m and length
of 225 m. Compute the
setback distance required
from the center line of the
inner side of the curve so
as to provide for Safe OSD
of 350 m

63. Gradient : Definition and Types
 Gradient shall be the rate of rise or fall. Gradient shall be
expressed as one of the following ways:
1. In percentage; example 10%,20%, 33% etc (n%)
 10% means the rise/fall of 10 units per 100 units of
horizontal distance travel .
2. in fraction; example 1in 40, 1 in 200, 1 in 2000 etc. (1
in N) 1 in 40 means the 1 unit of rise/fall (vertical dist.)
per 40 units of horizontal dist. travel.
 Based on Function, Gradient shall be of following 5 types
1. Ruling gradient: It is the maximum gradient within
which the designer attempts to design the vertical profile
of a road.
it is that maximum gradient over which a vehicle can be
hauled with one locomotive without the application of
additionally higher gears
The value of ruling gradient per IRC the recommended
value of ruling gradient is 1 in 30, 1 in 20 and 1 in 16.68 in
plane/rolling, mountainous and steep terrain.

64. 2. Limiting gradient
If ruling gradient requires huge amount of earthwork for e.g.
topography of a place has steeper gradients then in such case we
provide limiting gradient which is more than ruling gradient.
The length of limiting gradient is limited considering safety. The
limiting gradient is broken either by providing level road or road with
a ease gradient.
As per IRC the recommended value of limiting gradient is 1 in 20, 1
in 17.7 and 1 in 14.3 in plain/rolling, mountainous and steep terrain
respectively. It shall not be provided for lengths greater than 300m
and shall be followed by at least 300m of resting gradient.
3. Exceptional gradient
In some cases its quite impossible to provide ruling /limiting
gradient.
Under this condition exceptional gradient is provided.The length of
exceptional gradient is as less as possible. As per IRC the
recommended value of exceptional gradient is 1 in 15, 1 in 14.3 &
1 in 12.5 in plain/rolling, mountainous and steep slope.
The exceptional gradient shall not be provided more than 60m/km of
mild distance travelled and shall not be more than 100m at a time
and shall be provided with at least 300m of resting gradient

65. 4. Minimum gradient :
A minimum value of gradient is also required along the
direction of the road because of the drainage purpose. As
per IRC the minimum gradient of 0.5% to 1.0% should be
provided considering various situation.
5. Momentum Grade
If the vehicle has just travelled a downgrade then the
upgrade slightly higher than ruling gradient can be
provided without any compensations. The grade is such
that the inetrria gained by the vehicle is equal to the load
imposed by additional gradient
According to NRS
Ruling Gradient 7%
Limiting Gradient 10%
Exceptional Gradeint 12%

66. Definition and Types of Vertical Curve
 It is a curve provided between two tangents to ease
the change in gradient
 Necessity:
 i. to obtain adequate visibility and safe driving
 ii. to secure comfort to the passengers
 Types:
 i. Vertical Summit Curve
 ii. Vertical Valley Curve

67. Design of Summit Curve
 Length of Curve(L)
Decided so as to provide minimum sight distance (S)
L > S
𝐿 =
𝑁𝑆2
200( ℎ1+ ℎ2)2
L < S
𝐿 = 2𝑆 −
200( ℎ1+ ℎ2)2
𝑁
Where N = [g2-g1] = Change in grade
h1 = Height of Drivers eye from ground = 1.2m(unless
specified else)
h2 = Height of Obstruction = 0.15m(unless specified else)

68.  Position of Highest Point
𝑋ℎ =
𝐿
𝑁
∗ 𝑔1
 Tangential Correction at any point
T =
𝑁𝑥2
200𝐿

69. NUMERICAL EXAMPLE
A vertical summit curve is to be
designed when two grades +1/60 and -
1/45 meet each other at an RL of
1220.15m so that a ISD of 420m is to be
ensured calculate
a. Length of the curve
b. RL of the beginning and highest point
Assume h1 = 1.2m , h2 = 0.15m

70. Design of Valley Curve
 Length of Curve(L)
Decided so as to provide minimum sight distance (S) under
head lights
Case I : L > S
𝐿 =
𝑁𝑆2
200ℎ+200𝑆 𝑡𝑎𝑛α
Case II: L < S
𝐿 = 2𝑆 −
200ℎ+200𝑆 𝑡𝑎𝑛α
𝑁
Where N = [g2-g1] = Change in grade
h1 = headlight height = 0.75 m (unless specified else)
α = beam angle = 1® (unless specified else)

71.  Position of Lowest Point
𝑋𝑙 =
𝐿
𝑁
∗ 𝑔1
 Tangential Correction at any point
T =
𝑁𝑥2
200𝐿

72. NUMERICAL EXAMPLE
A valley curve is formed by descending
grade of 1 in 30 meeting an ascending
grade of 1 in 40. Design the length of
vertical curve to provide a sight
distance of 72 m
If the RL of the apex is 98.5m determine
the
(i) RL of BVC and EVC
(I )RL of the Lowest point

73. Grade Compensation

74. Factors affecting Selection of Grade
 Characteristics of dominating traffic
 Physical Factors like drainage , access to property ,
appearance , safety etc.
 Road Intersection , Bridge etc.
 Topography of the country

75. Criteria for alignment selection
 Horizontal alignment
 Long tangents
 Large Radius
 Shall avoid hair pin bend
 Shall avoid reverse curve
 Vertical alignment
 Long tangents (short tangents create saw tooth
arrangement which shall be avoided)
 Vertical curve shall be of high radius
 Vertical alignment shall blend well with the locality
 Shall avoid box cutting